What You Need to Know About Blocking Pads

By Christie Walker
The process to create a pair of eyeglasses is complex and requires a significant investment of time and resources, especially with the development of advanced coatings, such as super-hydrophobic. With the cost of a lens blank reaching upwards of 200 dollars, it is crucial for labs to protect their investments. LabTalk Magazine recently sat down with Geoffrey D. King, SecurEdge product manager, Saint-Gobain Performance Plastics, to discuss choosing the correct blocking pad for lens coatings and materials.

Why are edging pads so crucial to the lens manufacturing process?

Surprisingly, edging pads are much more complex than they appear. The edging process is rigorous and the pad is solely responsible for preventing lens slippage. As such, pads must withstand high torque forces applied to the lens during edging. Further, the pads’ adhesives must adhere to each lens without removing coatings or leaving residue. The foam core base of edging pads cannot be too stiff or it will strain the adhesive. On the other hand, foam substrates that are too soft can stretch too much, causing slippage. Each of these variables must be controlled to minimize waste and increase efficiency. Ultimately, though, edging pads that meet these needs can effectively maximize a lab’s ROI by protecting the upstream investment made in tracing, blocking and mapping the lens.

What are some of the leading causes of edging failure?

For a blocking pad to work properly, it must completely bond to both the lens and the edging block. Most often, edging failure occurs as a result of insufficient bond area of the blocking pad to the lens or poor wet-out of the adhesive. To ensure the pad completely conforms to the lens, a lab must properly match the block to the curvature of the lens and select blocking pads that are specially engineered for the lens material and coatings.

Do super-hydrophobic coatings affect the edging process?

Absolutely. Super-hydrophobic coatings are designed to repel fingerprints, dirt and water. As a result, adhesives also have difficulty bonding to these coatings. Using incorrect blocking pads can result in offset, stripped coatings or residue on the lens. To avoid these problems, many labs have turned to using supplementary film between pads and lens, increasing total systems costs. Products do exist, however, that are specially engineered to achieve precision edging of super-hydrophobic lenses. We tested many combinations of foam core bases and adhesives before the SecurEdge Plus product was commercialized. The advanced blocking pad features two high performance adhesives—one for the lens and one for the block—to minimize slippage and eliminate the need for supplementary film, reducing total systems costs. The flexible but strong foam core base provides superior torque resistance and conformability.

Can you share any application techniques for blocking pads?

Since conformability is often the culprit when slippage occurs, matching the pad to the lens and block curvature is crucial and will provide good wet-out. Additionally, ensure the lens is thoroughly clean before applying the pad as any contaminants will inhibit the bond strength. Most importantly, however, labs must match the adhesive to the lens coating and material. A wide range of edging pad solutions exist to meet the varying needs of labs. Along with SecurEdge Plus, Saint-Gobain’s full product line features SecurEdge, a blocking pad with a reinforced foam core base that is ideal for a variety of materials and coatings. The OP7 Series is an economical yet reliable blocking pad for general use.

To find an authorized SecurEdge distributor, please contact Geoff King at 1-800-724-0883.


Labtalk June 2020